Various systems using electromagnetic (“EM”) techniques have been known in the field of airborne subsurface geophysical surveying.
EM systems may come in various shapes and sizes, but they generally involve a source of electromagnetic energy (transmitter) and a receiver to detect the response of the ground. Generally speaking, geophysical EM methods involve the generation of a magnetic field by applying a periodic current to a transmitter coil system placed near the surface of the earth. This primary magnetic field induces electrical currents in the ground, and the secondary magnetic field produced by these currents is measured to provide information regarding ground conductivity distributions. By processing and interpreting the received signals, it is possible to make deduction about the distribution of conductivity in the subsurface.
A single or multi-turn loop is generally used as the transmitter element of EM systems. A time varying current passing through the loop can be used to create a time varying magnetic field. One or more receiver coils may be used to measure the response along perpendicular axes.
EM measurements can be done either in frequency domain or time domain. A frequency domain (FD) EM system transmits a magnetic field signal at a single frequency with sinusoidal variation in amplitude. The response can either be described by its total amplitude and phase with respect to the transmitter signal or by the amplitudes of components in-phase (“in-phase”) and 90° out of phase (“quadrature”) with respect to the transmitter signal. In time-domain (TD) EM systems, a pulse of current may be applied to the transmitter coil during an on-period, generating the primary or transmitted EM field, and then switched off during an off period. The secondary signal may be measured at the receiver coil as a function of time. The signal amplitude decay during the off-period, combined with modeling of the conductivity and geometry of geological bodies in the ground, may be utilized to provide conductivity contour maps.
EM methods can encompass both ground-based and airborne applications using airplanes and helicopters, etc.
In designing a helicopter mounted TDEM system, there are a number of desired features, for example, inter alia, high signal-to-noise ratio, high conductance discrimination, and high spatial resolution.
High signal-to-noise ratio can be accomplished by lowering system noise, and/or increasing the signal at the receiver coil. One method of increasing the signal is to increase the primary magnetic field. Stronger transmitter signal power, which can be generally obtained by using transmitter loops with large diameters, also assists in obtaining increased signal-to-noise ratio and greater penetration. One method of reducing or limiting noise is to reduce as much as possible the movement of the receiver relative to the transmitter. For example, rigid structures can be used to support and connect the transmitter and the receiver of an EM system.
In practice, however, the expected benefit of using a large transmitter loop can be difficult to obtain due to the non-rigidity or over-flexibility of the large transmitter loop structure, which distorts the shape of the transmitter loop during flight and the resulting primary magnetic field during airborne operation which in turn resulting in increased system noise. Therefore, the stabilization of the transmitter loop will provide less distortion of the receiver itself and better signal-to-noise ratio.
The increased weight and size associated with a large transmitter loop also impose mechanical challenges that impede the speed, reduce aerodynamics, and increase drag.
At the same time, using rigid structures existing in the prior art to couple the transmitter and the receiver, however, will increase the weight of the EM system, and therefore resulting in a heavy weight structure and increase the cost of survey operations. For example, it will be a mechanical challenge and may not be economical or practical for a helicopter to tow a transmitter weighing over 500 kg.
As a result, existing EM systems have not been able to satisfactorily take advantage of large transmitter loops. For example, AeroTEM II™ system is an airborne EM system having a rigid transmitter loop and a receiving coil that is mounted at the center of the transmitting loop. The transmitter and the receiver of the AeroTEM II™ system are supported and connected to each other using heavy rigid structures which limit its transmitter loop diameter to about 5 meter in order to have a manageable overall weight of the structure and not exceed tow capacity.
Another existing EM system described in Canadian Patent Application No. 2,702,346 proposed a large transmitter loop, where the polygonal receiver coil sits at the centre of the transmitter coil frame. However, the proposed transmitter loop has multiple “articulating joints” and is very flexible as it allows rotation of the frames relative to each other at the “articling joints” and as a result the structure can bend at a plurality of locations about a circumference of the transmitter loop. Therefore, none of these prior art EM systems provide a transmitter loop the size of which can be easily increased while maintaining the overall stability of the loop.
Therefore, there remains a need for an airborne EM system having a rigid supported transmitter loop that will maintain its stability as the size of the transmitter loop varies.